Pigment having angle dependence of the interference colors and its production process

ABSTRACT

The invention disclosed a pigment having angle dependence of the interference colors and its production process, in which mica is used as the substrate and the first metal oxide coating with high refractive index has the optical thickness of silver to golden interference color. And the second metal oxide coating with low refractive index has the optical thickness of the second circle green to the fourth circle interference color. The third coating is a highly refractive metal oxide. The lowly refractive metal oxide is SiO 2  and the highly refractive metal oxide is TiO 2 , SnO 2 , Fe 2 O 3 , Fe 3 O 4 , CoO, Co 2 O 3 , ZrO 2 , Cr 2 O 3  or their mixtures as well as complexes. The mica substrate is wet-milled mica powders with a thickness of 0.1-0.9 micrometers and a diameter of 5-250 micrometers. The process involves wet chemical hydrolysis steps to alternately deposit the coatings and modulating the optical thickness of each coatings, to produce the pigment having angle dependence of the interference colors, which has various hues and ranges of angle dependence of the interference colors as well as higher brightness and vivid colors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a National Stage Application of International Patent ApplicationNo. PCT/CN 2005/000520, with an international filing date of Apr. 18,2005, which is based on Chinese Patent Application No. 200410013060.3,filed Apr. 19, 2004. The contents of both of these specifications areincorporated herein by reference.

FIELD

This invention concerns pigments exhibiting color-shifting effects andthe synthesis thereof. Perl luster pigments consist of various chemicalraw materials and natural minerals and exhibit interference colorsdependent on angles of observation.

BACKGROUND

Because of their appreciation for beauty, people in different historicalperiods have developed various colorful pigments and dyestuffs tobeautify life. Luster pigments originated from pearls and then pearlluster pigments were developed. The early pearl pigments consisted ofguanine from natural fish scales. However, because of a limited amountand high price of natural pearl pigments, people have begun switching tosynthetic pearl luster pigments which fall into two major forms: one isa flake-like inorganic mixture of bismuth oxychloride and calcium-sodiumsilicates; the other is a multilayer pearl luster pigment formed bycoating metal oxides and non-metal oxides on the surface of a naturalwhite mica and synthetic mica with methods such as evaporating anddepositing, co-ion sputtering, and wet chemical methods. The reasons forwhy pearl luster pigments can produce pearl luster effects is that someof incident light is reflected, some is refracted, some is absorbed, andsome is refracted and reflected multiple times, which causes theformation of various interference colors and a pearl or rainbow-likeeffect.

The multilayered pearl luster pigments on a mica base cause thereflected or refracted light to exhibit different types of lusterdepending on the different grain diameters of the micas; for instance,small grain diameters can produce silk-like luster, while large graindiameters can produce glittering luster.

Because of the difference in thickness of the coating metal oxides, manyinterference colors are generated, such as, e.g., silver-white, yellow,orange, red, purple, blue and green, all of which are monochromatic.

The crystal-like synthetic calcium-sodium silicate is not applied inmany cases as the major pigment ingredient and is often used in smallamounts. Because of its low weather resistance and large specificgravity, the application range of the crystal-like synthetic bismuthoxychloride is limited.

The multilayered mica-based pearl luster pigment has excellent weatherand chemical resistance because of metal oxides having a high refractiveindex, such as titanium oxide and/or ferric oxide, coated on the micasurface. In addition, pigments with different grain diameters can beproduced through size classification. Therefore, this kind of pigment iswidely used in car coating, paints, coating, cosmetics, special wrappingpaper, decorative paper, plastics, plastic color agglomerates,artificial leather and various printing inks, etc. However, this pigmentcauses monochromatic light interference only and does not producecolor-shifting at various angles. It is only double-colored, if used asa dye, and thus cannot be used in certain fields and for certainapplications.

At present, only such companies as Warck, BSF, and Merck in Germany, andFlex in the United States can produce pigments having color-shiftingeffects. An analysis of the advantages and disadvantages of theirpatents is made hereinbelow.

Flex in the U.S. adopts opaque tinsel or metal oxides as the basematerials and coats the tinsel alternatively with metal oxides of highand low refractive indexes on one or both sides of the tinsel withvacuum-film plating. By controlling the optical thickness of the coatingmetal oxide layers, different color changes and color-shift zones can beobtained. The base is stripped from the net with certain meshes ordissolvable films. Therefore, precision instruments, stricttechnological processes, and highly pure chemical materials are neededresulting in high production cost and low productions. In addition, itis difficult to control the colors and color-shift zones, and a wide useof the pigment is limited because of its high price.

BSF in Germany adopts aluminum powders or synthetic ferric oxides (madeunder high temperature and pressure) as the base materials, the surfaceof which is coated alternatively with metal oxides of high and lowrefractive indexes using the VCD method. An inert gas transports boilingmetal particles and deposits them on the surface of the base material ata desired coating thickness. However, there are problems with effectivedispersing of the base in the cauldron and with dosing the amounts ofthe materials. Moreover, there is also the environmental problem ofusing organic metal compounds. In addition, the pigments are expensiveand limited in application because of complicated preparationprocedures, the necessity of precision instruments, the high cost ofbase making, the need for strict procedure control, and the high priceof the raw materials.

Merck in Germany adopts synthetic silicon dioxide as the base material(made by plating hydrate glass with certain density on a specialinstrument and drying and stripping it), and hydrolyses dissolvableinorganic metal and nonmetal compounds to produce hydrate metal oxidehaving high and low refractive indexes which are coated alternatively onthe base material. Because this is a wet chemical procedure, it is easyto operate and control the optical thickness of the desired coating. Inaddition, it is possible to obtain color-shifting effects by plating alayer of metal oxide having a high refractive index on the silicondioxide base surface. However, the thickness of the base must be lessthan 1 micron, usually between 200 and 500 nm, which causes difficultyfor the making of the base, high cost, low production, and the need forhigh precision instruments. Besides, the process of plating, drying, andstripping causes great wear and tear of the production machines andmakes the overall procedure complicated so as to result in a high costof base making. Consequently, the pigment is very expensive, whichlimits its application.

Warck in Germany adopts organic polymerized silicon liquid crystals ofcertain sizes, grinding them and sifting them out to pigments. As liquidcrystals, the pigments have a short life span and can not be used incertain applications. In addition, the high price of liquid crystalslimits their application.

U.S. Pat. No. 6,656,259 describes pigments exhibiting color-shiftingeffects and their synthesis. The pigments adopt flake-like silicondioxide as the base material whose surface is coated with single-layeredor multi-layered transparent or semi-transparent metal oxides.

U.S. Pat. No. 6,692,561 discloses multi-layered pigments exhibitingstrong optical interference and having mica as the base material. Thepatent alleges that the color of the pigments changes with the viewingangle and that the pigments glitter with interfering lights. The firstlayer coated on the mica base is a metal oxide with a low refractiveindex, but the optimal value of its size and its technical specificationare not stated. Therefore, it is alleged that the invention does notsolve the problem that it sets out to solve. Based on a statisticalanalysis, the specification disclosed wavelengths at a fixed viewingangle. As a result, the range of color-shifting of the pigment islimited, and the pigment belongs to the group of ordinary pearl lusterpigments, not reaching the full effect of color-shifting at differentviewing angles, for such pigment should be ascertained by multianglephotometer at least five times.

SUMMARY

The base of the pigment with a color-shifting effect is alternativelycoated with metal oxides of different refractive indexes and has anouter protective layer. The nature of the pigment lies in the basematerial of natural mica, the surface of which is coated with threelayers of metal oxides. The first layer is a metal oxide with highrefractive index used to smooth out the step-like mica surface. Thesecond layer is a metal oxide with a low refractive index. The thirdlayer is a metal oxide with a high refractive index.

The thickness of the second metal oxide layer having a low refractiveindex coated on the surface of the mica base is the same as or greaterthan the optical thickness of the 2^(nd)-order interference color.

The thickness of the first metal oxide layer having a high refractiveindex coated on the surface of the mica base is the optical thickness ofsilver-white to golden-yellow interference color.

The thickness of the second metal oxide layer having a low refractiveindex coated on the mica base is the optical thickness of from the2^(nd)-order green to the 4^(th)-order interference colors.

The second metal oxide with a low refractive index is SiO₂.

The metal oxides with high refractive indexes are TiO₂, SnO₂, Fe₂O₃,Fe₃O₄, CoO, Co₂O₃, ZrO₂, or Cr₂O₃ or their mixtures or theirderivatives.

The synthesis of the pigment is as follows: put wet grinded mica powderwith specific standards into deionized water and stir it into suspensionsolution. Heat it to 60-90° C. Then adjust its pH value to 2 to 9 withdiluted aqueous hydrochloric acid or sodium hydroxide solution. Add apre-measured amount of dissolvable inorganic salt solution having a highrefractive index. Keep the pH value constant by adding a hydrochloricacid or a sodium hydroxide solution. Deposit a hydrate metal oxide witha high refractive index at a desired thickness on the surface of themica. Stir it for 30 minutes at constant temperature for ageing. Adjustthe pH value to 6-14 with sodium hydroxide solution. Add a measuredamount of dissolvable inorganic salt solution having a low refractiveindex. Keep the pH value constant by adding hydrochloric acid solutionto make the metal oxide with a low refractive index deposit to a desiredthickness. Stir it for 30 minutes at a constant temperature for ageing.Adjust the pH value to 2 to 9 with aqueous diluted hydrochloric acid orsodium hydroxide solution. Add a pre-measured amount of inorganic saltsolution with a high refractive index. Keep the pH value constant byadding hydrochloric acid or sodium hydroxide solution to obtain thedesired color-shifting tones and zones. Stir it for 30 minutes at aconstant temperature for ageing. Filter, wash, dry, calcine, sift outand dry.

The dissolvable inorganic solution having a high refractive index is achlorine-containing compound, such as, e.g., TiCl₄, TiOCl₂, SnCl₄,SnCl₂, FeCl₃, FeCl₂, CoCl₂, ZrOCl₂ and CrCl₃.

The dissolvable inorganic metal salt solution having a low refractiveindex is a hydrate glass compound.

Organic or inorganic ferrous pigments or outer protection layers can becoated as additional layers.

The advantages of the invention are:

1. The pigment according to the invention adopts mica as the basematerial, which is easy to obtain and much cheaper than opaque tinsel,metal oxides, synthetic silicon dioxide and organic liquid crystalsilicon, thus greatly reducing the price of the pigment and broadeningits field of application.

2. Compared with U.S. Pat. No. 6,656,259 for a pearl luster pigmenthaving mica as the base material, the pigment of the present inventioncomprises as the first layer of its mica base metal oxides having a highrefractive index which allows for an easy reach of the desiredthickness. At the same time, the optimal thickness, which is a key toensure the brightness of color of the pigment, is defined as the opticalthickness of silver-white to golden-yellow.

3. Compared with U.S. Pat. No. 6,656,259 for pearl luster pigment withmica as the base material, the pigment of the present inventioncomprises in the second layer of the mica base metal oxides having a lowrefractive index whose thickness is the same as or greater than theoptical thickness of the 2^(nd)-order interference color. Particularly,the optical thickness between the 2^(nd)-order green and the4^(th)-order interference color is the technical key to produce variousinterfering colors. As experiments have proved, the thickness less thanthe optical thickness of the 2^(nd)-order green cannot produce acolor-shifting effect. However, this invention solves that problem.

4. The synthesis of the inventive pigment can meet the desiredthickness. It is possible to coat the mica surface alternatively withmetal oxides having high and low refractive indexes for one or moretimes. Thus it is possible to produce various color-shifting tones andzones which can be doubled or tripled or multiplied, covering allcolors.

5. The pigment can be manufactured using wet chemical hydrolysis andcoating. Using dissolvable inorganic metal compounds, it is easy tooperate without environmental pollution. Using wet chemical hydrolysis,it is also easy to control the coating thickness. In other words, bycontrolling the rate of pigment evaporation and deposition, it is easyto operate and control the quality of the products. Mica (or syntheticmica) is abundant for use as the base material of the invention. Inaddition, mica powders are low in price, nontoxic, and are easilyprocessed and produced at low cost, thus greatly reducing the cost ofthe pigments. The pigments can be widely used in paint, ink, plastics,cosmetics, pottery materials, printing inks, surface paint, in paper andglass-making, multiple paint spraying, in hydrate and solution systems,and in safety applications.

DETAILED DESCRIPTION

A pigment having a color-shifting effect comprising alternate layers ofmetal oxides with high and low refractive indexes, respectively,deposited via wet chemical hydrolysis on the surface of wet grindednatural mica having the thickness of 0.1-0.9 microns and the graindiameter of 5-250 microns. Through adjusting the optical thickness ofthe different layers, bright-colored pigments with a color-shiftingeffect with various zones of color and color change can be produced.

Being layer-structured, natural mica can be repeatedly cut into thinpieces theoretically until monocrystals are obtained. However, it isvery difficult and nearly impossible to actually obtain monocrystals inpractice. Usually, wet grinded mica powders are 0.1-0.9 microns thick,so it is inevitable to have mica grains with a rough surface, which maybe acceptable for making ordinary pearl luster pigment, but impracticalfor producing pigments having color-shifting effects. Therefore, it isnecessary to make improvements in coating structure.

The invention teaches depositing layers of metal oxides having a highrefractive index on the surface of mica powders so as to smooth itsstep-like surface and obtain uniform optical nature. Subsequently, itteaches directly coating hydrate SiO₂ to a desired thickness, and uponit coating other metal oxides with a high refractive index so as toobtain a color-shifting effect.

In some special cases, the pigments of the invention can be furtherprocessed or coated so as to meet additional requirements.

The basic procedure of the invention:

1. Pour wet grinded mica powders having specific standards intodeionized water and stir it into a suspension solution. Heat it to60-90° C., preferably 75-80° C. Then adjust the pH value to a desiredrange with diluted aqueous hydrochloric acid solution (e.g., pH 2.2,when TiO₂ is coated). Add a pre-measured amount of dissolvable inorganicsalt solution gaving a high refractive index. Keep the pH value constantby adding sodium hydroxide solution with a specific density to deposithydrate metal oxide having a high refractive index at a desiredthickness on the surface of the mica, reaching the optical thickness offrom silver-white to golden-yellow. Stir it for 30 minutes at constanttemperature.

2. Adjust the pH value to 6-12, preferably 6.5-9.5, with a sodiumhydroxide solution. Add a pre-measured amount of dissolvable inorganicsalt solution having a low refractive index. Keep the pH value constantby adding a hydrochloric acid solution. When the thickness of the secondlayer metal oxide is the same as or less than the optical thickness ofthe 2^(nd)-order interference color, and particularly the 2^(nd)-ordergreen, stir it for 30 minutes at constant temperature. Particularly,various colors can be obtained starting with the 2^(nd)-order green, andmore particularly, the range of the optical thickness is between the2^(nd)-order green and a 4^(th)-order interference color. The opticalthickness greater than the 4^(th)-order interference color can alsoproduce a color-shifting effect. However, this will increase the cost.

3. Adjust the pH value of the suspension solution to 2-5 with dilutedaqueous hydrochloric acid (according to the pH value of hydrolysis ofmetal oxides with a high refractive index, for example, when coated withFe₂O₃, pH value is 3.5). Add a pre-measured amount of inorganic metalsalt solution having a high refractive index to reach the desired tonesand zones of color change. Stir it for 30 minutes at constanttemperature. Filter, cleanse, dry, calcine, sift out and dry.

The calcination temperature is dependant on the coating material,usually 250-1000° C., and preferably 350-900° C. When coating pigment,it is accepted to separate, dry and calcine it after coating one layer,and then switch to another layer.

If needed, the finished pigment can be further processed or coated inorder to make it resistant against heat, light, weather, or chemicals,or to make it effectively attach to or detach from certain materials.

The pigment of the invention adopts mica as its base material whosesurface is alternatively coated with metal oxides having high and lowrefractive indexes, which can be done once or many times. Therefore, itis convenient and possible to produce various tones and zones ofcolor-shifting which can be doubled or tripled or multiplied, coveringall colors. The pigment of the invention finds applications incosmetics, car paint, coating, printing ink, decorative material,plastics, rubber, decorative wrapping paper, glass, pottery and allfields in which pearl luster pigment is used. At a low price, it iswidely used.

If inorganic or organic colored pigments such as ferrous metal oxides,e.g., needle iron ore, magnetite, hematite; chromium oxide; titanium lowoxide and chromium/iron compound; colored pigments, e.g., Berlin blue,Prussia blue, vanadium bismuth, chromium hydroxide, alumina cobalt,ultramarine, Tenard's Blue, sulfured cadmium, selenium compound,chromate pigments, and black carbon pigments; or organic coloredpigments, e.g., indigotin, thioindigo and their derivatives, nitricpigments, benzimidazole, anthraquinone, indanthrene dye, dihydroxyquinoline, acridine; metal sulfide; hydrous metal sulfide and magenta;are further coated on the outer side of the metal oxides or BiOCl, thecolor of the pigment powders will have an obvious change and produce aninteresting color-shifting effect.

The pigment of the invention can be conveniently blended in anyproportion with organic dyes, other inorganic pigments and otherinorganic single or multiple-layered pigments such as the traditionalpearl luster pigments and LCPs based on phyllosilicate, glass, SiO₂ ormetal base as well as the traditional adhesives and fittings.

The metal oxides having a high refractive index are TiO₂, SnO₂, Fe₂O₃,Fe₃O₄, CoO, Co₂O₃, ZrO₂, or Cr₂O₃ or their mixture or compounds.

The metal oxides with a low refractive index are SiO₂, AlCl₃, AlCl₃Al(OH)₃, B₂O₃ or their mixture or compounds

The dissolvable inorganic salts which can hydrolyze into metal oxideswith a high refractive index are chlorine-containing compounds, such as,TiCl₄, TiOCl₂, SnCl₄, SnCl₂, FeCl₃, FeCl₂, CoCl₂, ZrOCl₂ and CrCl₃, etc.

The dissolvable inorganic salts which are hydrolyzed into metal oxideshaving low refractive indexes are water glass, sodium silicate, AlCl₃,NaAlO₂, borax, etc.

The flake-like materials that can be used as base material in theinvention are various flake-like materials, including natural andartificially made flake-like materials, such as, e.g., natural mica,synthetic mica, natural mica iron, flake-like Fe₂O₃, graphite scales,mica sequential silver-white or golden-yellow pearl luster pigment. Themain material is mica, such as, white mica, golden mica, silk mica,black mica, with high-quality white mica.

The white mica powder adopted in the invention is calcined from naturalwhite mica under high temperature with the addition of oxygen, andwet-grinded and processed chemically so as to obtain mica powders withdesired grain diameter and thickness of the base.

The base materials of the invention can also be obtained directly by wetgrinding and chemical processing without calcining under hightemperature so as to obtain mica powders with desired grain diameter andthickness as the base.

The main structure of the pigment of the invention is symmetrical, suchas, e.g.,

Fe₂O₃/SiO₂/Fe₂O₃/mica/Fe₂O₃/SiO₂/Fe₂O₃;

TiO₂/SiO₂/TiO₂/mica/TiO₂/SiO₂/TiO₂;

Cr₂O₃/TiO₂/SiO₂ TiO₂/mica/TiO₂/SiO₂/TiO₂/Cr₂O₃;

TiO₂/SiO₂/Fe₂O₃/mica/Fe₂O₃ SiO₂/TiO₂;

Fe₂O₃/SiO₂/TiO₂/mica/TiO₂/SiO₂/Fe₂O₃;

Fe₂O₃.TiO₂/SiO₂/Fe₂O₃.TiO₂/mica/Fe₂O₃.TiO₂/SiO₂/Fe₂O₃.TiO₂;

TiO₂/Fe₂O₃/SiO₂/Fe₂O₃/TiO₂/mica/TiO₂/Fe₂O₃/SiO₂/Fe₂O₃/TiO₂;

TiO₂/SnO₂/TiO₂/SiO₂/TiO₂/SnO₂/TiO₂/SiO₂/mica/TiO₂/SnO₂/TiO₂/SiO₂/TiO₂/SnO₂/TiO₂/SiO₂;

Fe₂O₃/SiO₂/Fe₂O₃/SiO₂/Fe₂O₃/SiO₂/Fe₂O₃/mica/Fe₂O₃/SiO₂/Fe₂O₃/SiO₂;

Fe₂O₃/SiO₂/Fe₂O₃;

TiO₂/CoO/SiO₂/CoO/TiO₂/mica/TiO₂/CoO/SiO₂/CoO/TiO₂; or

CoO/Fe₂O₃/SiO₂/CoO/Fe₂O₃/mica/Fe₂O₃/CoO/SiO₂/Fe₂O₃/CoO.

The structures given above as well as other structures are within thescope of the invention.

The invention can produce multiple repeating layers based on the basicstructures above. That is the mica surface is multiply coated with metaloxides having high and low refractive indexes so as to obtain the neededtones and color-shifting effect, as follows:

—Fe₂O₃/SiO₂/Fe₂O₃/SiO₂/Fe₂O₃/SiO₂/Fe₂O₃/mica/Fe₂O₃/SiO₂/Fe₂O₃/SiO₂/Fe₂O₃/SiO₂/Fe₂O₃—;

—TiO₂/SiO₂/TiO₂/SiO₂/TiO₂/SiO₂/TiO₂/mica/TiO₂/SiO₂/TiO₂/SiO₂/TiO₂/SiO₂/TiO₂—

The structure above is included in the invention and can be coatedmultiple times.

Examples will now follow illustrating the production procedure of thepigment of the invention.

EXAMPLE 1

The structure of the pigment is Fe₂O₃/SiO₂/Fe₂O₃/mica/Fe₂O₃/SiO₂/Fe₂O₃;

Suspend 16 g white mica powders with a grain diameter of 10-60 micronsin 500 mL deionized water and stir and heat it to 75° C.

Adjust the pH value of the suspension to 3.5, add 18% hydrochloric acidas well as pre-measured 300 mL of 10% FeCl₃ solution. At the same timekeep the pH value constant by adding 15% Na₃OCl dropwise, and at laststir it at constant temperature for 15 minutes.

Adjust the pH value of the suspension to 9.5 with 32% sodium hydroxidesolution and stir it for 15 minutes.

Add 2 L of a sodium silicate solution (7 g/l SiO₂) dropwise, at the sametime keep pH value at 9.5 by adding 15% hydrochloric acid dropwise, andat last stir it for 30 minutes.

Adjust the pH value of the suspension to 3.5 with 18% hydrochloric acidsolution and stir it for 30 minutes. Add 70 mL of a sodium silicatesolution (120 g/l FeCl₃₎) dropwise, and at the same time keep pH valueconstant by adding 15% hydrochloric acid dropwise and at last stir itfor 15 minutes.

The product is filtered, cleansed, dried, calcined at 500° C., sieved100 times, and dried. It is then blended with certain proportion ofadhesives made of hydrate oxidized starch and PVA, and scraped withscraping paper having white and black bottom.

The color varies between purplish-red and yellow-green as observed atvertical-horizontal angles of viewing.

The core of the invention lies in its base material of natural micawhose surface is coated with three-layered metal oxides. The first layeris the metal oxide with high refractive index. The second tire is themetal oxide with low refractive index. The third tire is the metal oxidewith high refractive index. Particularly, the thickness of the firstlayer is the optical thickness of silver white to golden yellow, and thethickness of the second layer is the same as or greater than the opticalthickness of the 2^(nd)-order interference color. No matter what kind ofmaterial is adopted, how many layers are coated, or how the thicknessvaries, all such variations are within the protection of the invention.

1. A pigment exhibiting a color-shifting effect comprising a basematerial; a first layer having a first optical thickness; a second layerhaving a second optical thickness; a third layer; and optionally anouter protective layer, wherein said base material is a mica; said firstlayer and said third layer each independently comprise a metal oxidehaving a high refractive index; said second layer comprises a metaloxide having a low refractive index; said base material is coated with(1) said first layer, (2) said second layer, (3) said third layer, andoptionally (4) said outer protective layer, said first layer being indirect contact with said base material, said second layer being indirect contact with said first layer and said third layer, and saidprotective layer being in direct contact with said third layer; saidfirst optical thickness is greater than an optical thickness ofsilver-white interference color and smaller than an optical thickness ofgolden-yellow interference color; and the pigment exhibits acolor-shifting effect.
 2. The pigment of claim 1, wherein said secondoptical thickness is greater than an optical thickness of a 2^(nd)-orderinterference color.
 3. The pigment of claim 1, wherein said secondoptical thickness is greater than an optical thickness of a 2^(nd)-ordergreen interference color and smaller than an optical thickness of a4^(th)-order interference color.
 4. The pigment of claim 1, wherein saidmetal oxide having a low refractive index is SiO₂.
 5. The pigment ofclaim 1, wherein said metal oxide having a high refractive index isselected from TiO₂, SnO₂, Fe₂O₃, Fe₃O₄, CoO, Co₂O₃, ZrO₂, Cr₂O₃, andmixtures and derivatives thereof.
 6. The pigment of claim 1, whereinsaid outer protective layer is an organic or an inorganic ferrouspigment.
 7. A pigment exhibiting a color-shifting effect comprising abase material; a first layer having a first optical thickness; a secondlayer having a second optical thickness; a third layer; and optionallyan outer protective layer, wherein said base material is a mica; saidfirst layer and said third layer each independently comprise a metaloxide having a high refractive index; said second layer comprises ametal oxide having a low refractive index; said base material is coatedwith (1) said first layer, (2) said second layer, (3) said third layer,and optionally (4) said outer protective layer, said first layer beingin direct contact with said base material, said second layer being indirect contact with said first layer and said third layer, and saidprotective layer being in direct contact with said third layer; saidfirst optical thickness is greater than an optical thickness ofsilver-white interference color and smaller than an optical thickness ofgolden-yellow interference color; said second optical thickness isgreater than an optical thickness of a second order interference color;and the pigment exhibits a color-shifting effect.
 8. The pigment ofclaim 7, wherein said metal oxide having a low refractive index is SiO₂.9. The pigment of claim 7, wherein said metal oxide having a highrefractive index is selected from TiO₂, SnO₂, Fe₂O₃, Fe₃O₄, CoO, Co₂O₃,ZrO₂, Cr₂O₃, and mixtures and derivatives thereof.
 10. The pigment ofclaim 7, wherein said outer protective layer is an organic or aninorganic ferrous pigment.
 11. A pigment exhibiting a color-shiftingeffect comprising a base material; a first layer having a first opticalthickness; a second layer having a second optical thickness; a thirdlayer; and optionally an outer protective layer, wherein said basematerial is a mica; said first layer and said third layer eachindependently comprise a metal oxide having a high refractive index;said second layer comprises a metal oxide having a low refractive index;said base material is coated with (1) said first layer, (2) said secondlayer, (3) said third layer, and optionally (4) said outer protectivelayer, said first layer being in direct contact with said base material,said second layer being in direct contact with said first layer and saidthird layer, and said protective layer being in direct contact with saidthird layer; said first optical thickness is greater than an opticalthickness of silver-white interference color and smaller than an opticalthickness of golden-yellow interference color; said second opticalthickness is greater than an optical thickness of a second order greeninterference color and smaller than an optical thickness of a fourthorder interference color; and the pigment exhibits a color-shiftingeffect.
 12. The pigment of claim 11, wherein said metal oxide having alow refractive index is SiO₂.
 13. The pigment of claim 11, wherein saidmetal oxide having a high refractive index is selected from TiO₂, SnO₂,Fe₂O₃, Fe₃O₄, CoO, Co₂O₃, ZrO₂, Cr₂O₃, and mixtures and derivativesthereof.
 14. The pigment of claim 11, wherein said outer protectivelayer is an organic or an inorganic ferrous pigment.
 15. A method ofpreparing a pigment exhibiting a color-shifting effect of claim 11, themethod comprising the steps of (i) heating mica powder suspension inwater; (ii) adjusting the pH value of the mixture to between 2 and 9 byadding a diluted aqueous hydrochloric acid solution or a diluted aqueoussodium hydroxide solution; (iii) adding a first inorganic salt solution;(iv) maintaining the pH value of the mixture constant by adding adiluted aqueous hydrochloric acid solution or a diluted aqueous sodiumhydroxide solution; (v) stiffing the mixture at constant temperature;(vi) adjusting the pH value of the mixture to between 6 and 14 by addinga diluted aqueous hydrochloric acid solution or a diluted aqueous sodiumhydroxide solution; (vii) adding a second inorganic salt solution;(viii) maintaining the pH value of the mixture constant by adding adiluted aqueous hydrochloric acid solution or a diluted aqueous sodiumhydroxide solution; (ix) stirring the mixture at constant temperature;(x) adjusting the pH value of the mixture to between 2 and 9 by adding adiluted aqueous hydrochloric acid solution or a diluted aqueous sodiumhydroxide solution; (xi) adding a first inorganic salt solution; (xii)maintaining the pH value of the mixture constant by adding a dilutedaqueous hydrochloric acid solution or a diluted aqueous sodium hydroxidesolution; and (xiii) stirring the mixture at constant temperature. 16.The method of claim 15, wherein said first inorganic salt solutioncomprises one or more compounds selected from the group consisting ofTiCl₄, TiOCl₂, SnCl₄, SnCl₂, FeCl₃, FeCl₂, CoCl₂, ZrOCl₂ and CrCl₃. 17.The method of claim 15, wherein said second inorganic salt solutioncomprises sodium silicate.
 18. The method of claim 15, wherein in step(i) the temperature is maintained at 60-90° C.
 19. The method of claim15, wherein in steps (v), (ix), and (xiii) the mixture is stirred atconstant temperature for a period of about 30 minutes.
 20. The method ofclaim 15 comprising further as step (xiv) flittering of the pigment,washing it, drying it, and optionally calcining it.